NOVEMBEB 1, 1898.] 



KNOWLEDGE, 



261 



by no means constant. As a general rule one may say 

 that the " homosporous " condition in the ferns is always 

 associated with an independent prothallus, producing both 

 archegonia and antheridia. Here we have large special 

 spores that produce rudimentary prothalli bearing egg- 

 cells. 



It is natural, therefore, to turn to the microspores to 

 discover the origin of the spermatoids which fertilize these 

 egg-cells ; and it is not difficult to assure oneself that such 

 spermatozoids are developed from the microspores. A 

 very close study of the changes taking place within these 



It remains merely as a vestige — a remnant of the ancestral 

 cell-tissue on which the antheridia were formed. 



The spermatozoids are set free by the breaking of the 

 spore-waO, and the development of the embryo-plants 

 resulting from their contact with the egg-cells can be 

 observed in sections of prothaUi in their later stages. The 

 dividing egg-cell soon becomes a definite cell-tissue, in 

 which can be distinguished the young stem growing-point 

 with its first-formed leaves, the commencing root, and a 

 row of cells known as the xuspensor, which is of interest as 

 occurring here and in higher plants, but not in other 



A. — Macrospore of Selaginella. spinulosa, A. Br. B. — Microspores of the same species, c. and D. — Stages in the 

 division of a Microspore : the mother-cells of the Spermatozoids in the centre, the " Vegetative," or "' Prothallus " Cell at the 

 lower pole. E. — Spermatozoids. (Highly magniBed.) f. — Section of a Macrospore after germination, showing the Cellular 

 Mass (Prothallus) produced by the sub-division and outgrowth of its contents. On the right side above is an unfertilized 

 Archegonium, and on the left side a developing embryo, resulting from fertilization of a similar organ, a. — Later stage of an 

 Embryonic Plant. The .ipical growing point, with a pair of young leaves and ligules, to the left. H. — A young Sporophyte of 

 S. helvetica. I. K. — -Procumbent and ascending portions of the mature Sporophyte of 5. spinulosa. The terminal leaves of 

 the up-growing shoots bear the Sporangia at their bases, and are crowded together to form a spike or cone. L and ii. — Detached 

 leaves from the fertile spike, showing the Macro- and Micro-Sporangia, entire and dehiscent, x. — Part of a section through a 

 fertile spike of 5^. spinulosa. showing in different stages of development the Microsporangia above and Macrosporangia below. 

 [Fig. c, D, E, after Belajeff ; r and &, after Pfeffer ; ir, after Bischoff. The rest original.] 



spores is necessary to a full understanding of their nature, 

 and requires more elaborate observation. Such examina- 

 tion leads, however, to this conclusion : that the whole 

 contents of the microspore are not used up in the formation 

 of the spermatozoid. The early division of the microspore 

 contents shuts oil" a portion, the so-called " vegetative cell," 

 which seems to be of no further use. There is stiO, that 

 is to say, inherent in the spore, the tendency to develop 

 a prothallua-tissue, but this never grows to any extent. 



cryptogams. The details of development of this little 

 embryo from the egg-cell are very complicated, but for our 

 present purpose we need only note that it can be traced 

 through its various stages to such a form as Fig. H, and 

 that this in turn grows into the Selaginella plant. 



The question of the origin of the heterosporous type is 

 fall of interest, but we have little evidence to indicate the 

 lines of its evolution. It is impossible to say whether a 

 tendency to differentiation of the spores into two kinds 



